Yesterday, Sir Ian Blair, the Metropolitan Police Commissioner, expressed his frustration that when the police arrest people who carry knives in public, so few of them are sent to prison by the courts. The Met analysed the cases of 103 individuals caught with knives during one week in June. Of the 53 of them who had been sentenced by 28 July, one of 24 youths was sent to prison; of the 29 adults sentenced, eight were given custodial sentences, one for just a day. In other words, just 17 per cent of offenders were being jailed.

Granted that the remaining 50 of those originally arrested were still awaiting sentence or trial for more serious knife-related offences and a greater proportion of them may well go to prison. But Sir Ian is right to conclude that people who carry knives can assume they will not be imprisoned. This is the opposite of what he says should be the case. Obviously, as Sir Ian admits, it is not feasible that every offender will be imprisoned; the numbers are just too great. But if the culture of knife violence is to be tackled seriously, then the threat of a custodial sentence must be real. If the police fulfil their side of the bargain with the public and catch knife-carriers, the courts must fulfil theirs, and send more of those convicted to prison. The alternatives to prison - referral to a youth offender panel or community orders or tagging and curfew orders - are less effective and more variable.

The obvious problem is that prisons are packed to capacity. It is difficult for the courts simultaneously to fulfil Home Office recommendations for giving fewer custodial sentences and to respond to the public demand that knife crime should be punished. But providing sufficient prison space is the job of government - this government has failed to do so.

Knife crime has to be tackled by engaging with the underlying social problems as well as increasing arrest rates for offenders. But it can never be brought under control unless teenagers who set out to carry knives know they face prison if they do.

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